Methods for producing dynamo-electric machine armatures with improved balance

ABSTRACT

Armatures for dynamo-electric machines are balanced during manufacture by measuring the unbalance of the armature assembly prior to winding the coils on the armature. The numbers of turns of wire in at least some of the coils subsequently wound on the armature are then adjusted so that the unbalance of the resulting coils compensates for the unbalance of the armature prior to coil winding. In addition, masses may be added to the armature to ensure that it is balanced dynamically as well as statically.

BACKGROUND OF THE INVENTION

This invention relates to methods and apparatus for producing armaturesfor dynamo-electric machines such as electric motors and generators, andmore particularly to improving the balance of such armatures.

The principal components of a dynamo-electric machine armature aretypically a shaft, an axially slotted lamination stack or core mountedconcentrically on the shaft, a commutator also mounted concentrically onthe shaft, insulating end fibers at respective opposite axial ends ofthe lamination stack, insulating papers in the slots in the laminationstack, coils of wire wound on the lamination stack chiefly by passingthrough the slots in the stack with coil lead wires extending to thecommutator, and a resin coating applied to at least the axial ends ofthe coils to help stabilize the coils.

It is becoming increasingly important for such armatures to be wellbalanced about the central longitudinal axis of the armature shaft. Thisincreased importance is due, for example, to a growing interest (on thepart of motor manufacturers and users) in motors that operate moresmoothly, more reliably, with longer lives, and at higher speeds. Thetraditional techniques for balancing armatures include subjecting theannular outer surface of the lamination stack to a turning operation toensure concentricity of that surface with the shaft, milling one or moreaxial grooves in the outer surface of the lamination stack to removematerial from the side of the armature found to be heavier, and/oradding extra resin to the coil ends on the side of the armature found tobe lighter. It would be desirable, however, to assemble the armature insuch a way that unbalance is eliminated or at least substantiallyreduced so that the required extent of the above-mentioned traditionalbalancing operations can be at least substantially reduced. For example,removal of large amounts of material from the outer surface of thelamination stack by annular turning or axial milling may reduce theefficiency of the resulting motor. Also, to the extent that differentamounts of material must be removed from different armatures, thesetechniques are not consistent with producing motors having uniformoperating characteristics.

In view of the foregoing, it is an object of this invention to providedynamo-electric machine armatures with improved balance.

It is a more particular object of this invention to provide methods forassembling dynamo-electric machine armatures in such a way that theirbalance is improved prior to the manufacturing stage in which finalbalancing operations are traditionally performed so that the extent towhich such traditional final balancing operations must be carried out isat least substantially reduced.

SUMMARY OF THE INVENTION

These and other objects of the invention are accomplished in accordancewith the principles of the invention by measuring the unbalance of thearmature prior to the coil winding operation, and then winding the coilsof the armature so that the coils are unbalanced in a way thatcompensates for the previously measured unbalance. For example, thedirection and magnitude of unbalance prior to coil winding may bemeasured. Then the coils wound around a diameter of the lamination stackwhich is aligned with the unbalance direction may be wound so that thecoil on the side of the armature opposite the unbalance direction hasmore turns of wire than the parallel coil on the other side of thearmature. The difference in the number of turns of wire between thesetwo coils may be such that the wire mass difference times the radialdistance from the center of the armature through which that massdifference acts is equal to the magnitude of the unbalance of thearmature measured prior to coil winding. Because the direction of coilunbalance is opposite the direction of unbalance prior to coil winding,the deliberately unbalanced winding substantially cancels out theunbalance prior to winding, thereby producing a fully assembled armaturewhich requires little or no final balancing such as by axial milling ofthe outer surface of the lamination stack.

In addition to ensuring static balance of the armature as describedabove, dynamic balance may be achieved by adding masses to the armaturein such a way as to cancel out any dynamic unbalance. For example,masses of a dense resinous gel may be added to the axial ends of thecoils to eliminate or at least substantially reduce any dynamicunbalance. The magnitudes of these masses and their radial and axiallocations on the armature are preferably chosen so that they do notaffect the static balance of the armature but so that they are effectiveto counteract the dynamic unbalance of the armature.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of an early stage in the assembly of atypical armature for a dynamo-electric machine.

FIG. 2 is a view similar to FIG. 1 showing a subsequent stage in theassembly of a typical dynamo-electric machine armature.

FIG. 3 is another view similar to FIG. 1 showing a further subsequentstage in the assembly of a typical dynamo-electric machine armature.

FIG. 4 is still another view similar to FIG. 1 showing a still furthersubsequent stage in the assembly of a typical dynamo-electric machinearmature.

FIG. 5 is a simplified cross sectional view of the armature assemblyshown in FIG. 4. FIG. 5 includes a vector diagram useful in explainingthe principles of this invention.

FIG. 6 is a simplified elevational view, partly in section, ofillustrative armature unbalance measuring apparatus which can beemployed in the practice of this invention.

FIG. 7 is another simplified elevational view of the apparatus shown inFIG. 6. FIG. 7 is taken from the left in FIG. 6, and shows some elementsschematically.

FIG. 8 is a partly schematic, simplified, elevational view ofillustrative armature winding apparatus which can be employed in thepractice of this invention.

FIG. 9 is a view similar to FIG. 5, but shows the armature partly woundin accordance with this invention.

FIG. 10 is a fragmentary isometric view of an armature wound inaccordance with this invention, together with apparatus for ensuringthat the armature is dynamically balanced.

FIG. 11 is a simplified flow chart of illustrative steps for making anarmature in accordance with this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Assembly of a typical armature prior to winding is shown in FIGS. 1-4.In FIG. 1 a stack 14 of laminations of ferromagnetic material is pressedonto armature shaft 12. In FIG. 2 insulating end fibers 16a and 16b arepressed onto shaft 12 against respective opposite axial ends oflamination stack 14. Insulating papers (not shown) may also be insertedinto the axial slots 18 in lamination stack 14. In FIG. 3 commutator 20is pressed onto one axial end of shaft 12. The armature assembly 10ready for winding is shown in FIG. 4.

The armature structure shown in FIG. 4 may be unbalanced about thecentral longitudinal axis 22 of shaft 12 as a result of any of severalfactors. The production of shaft 12 requires lathing and grinding asteel bar with extremely precise tolerances. Once the shaft has beenproduced, it is subjected to a straightening operation and is thenplaced within lamination stack 14 as shown in FIG. 1. By itself shaft 12typically has relatively little unbalance about axis 22, although someunbalance of the shaft is always possible.

Lamination stack 14 is produced by punching individual laminations fromsteel sheets, and then assembling a stack of such individuallaminations. The individual laminations may not be perfectly balanceddue to variations in the thickness of the sheet from which a givenlamination is cut, imprecision in the shape of the lamination, etc. Theunbalance vectors of the individual laminations in stack 14 sum togetherto produce the net unbalance vector of stack 14. This may be the singlelargest source of unbalance in armature assembly 10, especially becausestack 14 is typically the component of assembly 10 with the largestdiameter and the greatest mass. The substantially cylindrical externalsurface of lamination stack 14 may be subjected to a lathe turningoperation to ensure concentricity of that surface with axis 22. Butstack 14 may still be significantly unbalanced about axis 22 due to suchfactors as nonuniform sheet thickness, imprecise lamination shape, etc.

In end fibers 16 imperfect mass distribution, imprecise geometry, andlack of concentricity with axis 22 may be another source of unbalance inassembly 10.

Still another source of unbalance in assembly 10 may be commutator 20,which may have less than perfect concentricity with shaft 12 or lessthan perfect mass distribution about axis 22. Commutator 20 may besubjected to a lathe turning operation to ensure concentricity of itssubstantially cylindrical external surface with axis 22, but imperfectmass distribution in the commutator may not be completely eliminated bythis operation.

FIG. 5 shows a typical net unbalance vector R which may result fromseveral individual unbalance sources such as are described above. Inparticular, vector R (having angle theta from radial reference axis 24)is the vector sum of individual unbalance components R₁, R₂, and R₃(having angles theta₁, theta₂, and theta₃, respectively, from referenceaxis 24). Unbalance component R₁ may be due, for example, to laminationstack 14, unbalance component R₂ may be due to commutator 20, andunbalance component R₃ may be due to end fibers 16. As shown in FIG. 5static balance can be restored to assembly 10 by adding anotherunbalance component F to the assembly, component F being equal andopposite to vector R.

In accordance with the present invention, the magnitude and direction ofnet unbalance vector R are determined. This can be accomplished, forexample, by placing assembly 10 in unbalance measuring apparatus 100 ofthe type shown in FIGS. 6 and 7. This apparatus, which may beconventional and commercially available, includes floating supports 102aand 102b for supporting the respective opposite axial ends of shaft 12via idle wheels 104. Assembly 10 is then rotated about axis 22 by amotor-driven belt 106 which is pressed against the outer cylindricalsurface of lamination stack 14. Any unbalance of assembly 10 causessupports 102 to reciprocate (as indicated by the double-headed arrow 108in FIG. 7) as assembly 10 is thus rotated. Sensor 110 detects thisreciprocation and applies a corresponding signal to the controls 120 ofthe machine. At the same time sensor 112 periodically reads a referencemark which has been placed at a particular angular location on thecylindrical surface of commutator 20. (Alternatively, sensor 112 maykeep track of the angular location of the armature by detecting featuresof the commutator such as its bars 21a or tangs 2lb.) The output signalof sensor 112 is also applied to machine controls 120. Controls 120 cantherefore determine the magnitude and direction (theta) of unbalancevector R. Vector magnitude is determined from the amplitude of thereciprocation detected by sensor 110. Vector direction is determinedfrom the phase relationship between the reciprocation detected by sensor110 and the reference mark (or other angular orientation) readingsproduced by sensor 112.

If desired, the values of vector R magnitude and direction determined asdescribed above can be marked (in encoded form) on assembly 10 by amarking tool 130 driven by controls 120. Alternatively, any othersuitable technique can be used for associating with assembly 10 thevalues of vector R magnitude and direction determined by apparatus 100so that those values are available for future reference during thesubsequent processing of that particular assembly. In the depictedembodiment, however, it will be assumed that marking tool 130 is used torecord the vector magnitude and direction on the outer cylindricalsurface of lamination stack 14 at a particular angular location (e.g.,at a predetermined angular location relative to the reference mark readby sensor 112 or, alternatively, at a predetermined location relative tothe direction of vector R).

After the magnitude and direction of net unbalance vector R have beendetermined as described above, assembly 10 may be placed in a coilwinding machine 200 such as is shown in FIG. 8. In the particularembodiment shown in the drawings, coil winding machine 200 is aconventional dual-flyer type winding machine. It will be understood,however, that any other type of coil winding machine (e.g., machines ofthe type shown in commonly assigned, co-pending application Ser. No.07/738,199) can be used instead if desired. In accordance with thisinvention, coil winding machine 200 is augmented by sensor 220 forreading from the cylindrical outer surface of lamination stack 14 theunbalance vector data previously inscribed there by marking tool 130 inFIGS. 6 and 7. The controls 202 of winding machine 200 use the machine'sconventional armature rotating components 204 to rotate assembly 10until sensor 220 can read the unbalance vector data from assembly 10.

Winding machine controls 202 use the unbalance vector data read bysensor 220 to modify the coil winding process to compensate for theunbalance of assembly 10. As shown in FIG. 9, for example, windingmachine may compensate for unbalance vector R by winding coil 30b withsufficiently more mass than coil 30a so that the unbalance vector Fwhich results from this coil unbalance is equal and opposite to vectorR. As is conventional, coils 30a and 30b are diametrically opposite toone another in diametrically opposite pairs of slots 18 in laminationstack 14. Coils 30a and 30b are wound at substantially the same time byflyers 210a and 210b, respectively. Flyers 210a and 210b arerespectively rotated about axis 206 by conventional flyer rotatingcomponents 208a and 208b controlled by controls 202.

Normally coils 30a and 30b would have the same number of turns and wouldbe of the same size. Coils 30a and 30b would therefore normally bebalanced about armature axis 22. However, in accordance with thisinvention, winding machine controls 202 cause flyer 210b to apply moreturns of wire to coil 30b than flyer 210a applies to coil 30a. Controls202 calculate the number of turns of wire by which coil 30b must differfrom coil 30a in order to substantially compensate for vector R. Forexample if the parallel planes in which coils 30 lie are spaced apart bya perpendicular distance 2d, and if the mass of a turn of wire in eachcoil is m, then the difference n in number of wire turns between coils30b and 30a required to offset unbalance vector R is given by theequation:

    n=R/md                                                     (1),

where R in this equation is the magnitude of unbalance vector R. Ifwinding machine 200 winds coil 30b with n more turns than coil 30a, thencoils 30 will have an unbalance vector whose magnitude is given theequation:

    F=nmd                                                      (2).

Because coils 30 are wound in planes that are perpendicular to vector Rwith the larger coil 30b in the direction away from vector R, vector Fis directed away from vector R. Equal and opposite vectors F and Rcancel one another and restore static balance to armature 10.

It will be appreciated that winding machine controls 202 determine notonly the number of turns by which coils 30a and 30b must differ from oneanother, but also the angular position at which coils must be wound onlamination stack 14 so that coil unbalance vector F is directedoppositely from vector R. This can be done in any of several ways. Forexample, the apparatus of FIGS. 6 and 7 can rotate assembly 10 so thatmarking tool 130 always marks the outer surface of lamination stack 14at a location which is 90° counterclockwise from vector R as viewed fromcommutator 20. Assuming that the end of assembly 10 visible in FIG. 8 isthe commutator end, winding machine controls 202 will know that whensensor 220 can read the unbalance markings from tool 130, the coils itis about to wind on assembly will be in planes perpendicular to vectorR, and that these are therefore the coils to be modified to compensatefor R. Moreover, controls 202 will know that (like coil 30b in FIG. 9)the coil on the left is to be made larger than the coil on the right.The only parameter controls 202 must then compute is n as in equation(1) above. Of course many other techniques can be used to enablecontrols 202 to determine which coils to modify to produce anappropriate coil unbalance vector F.

In the example shown in FIG. 9 only one pair of coils must be modifiedto produce a vector F of appropriate magnitude and direction. In thisexample all other coil pairs (either wound before or after coils 30) canbe wound in the conventionally balanced manner. If lamination stack 14were configured differently, or if vector R were directed differently(e.g., along one of the arms of stack 14 in FIG. 9), it might benecessary to modify two pairs of coils in the manner described above inorder to produce a net coil unbalance vector F directed oppositely tovector R. It will be readily apparent to those skilled in the art howthis capability can be included.

It may also be desirable to have winding machine 200 base the angularorientation of all the coils to be wound on assembly 10 on the directionof unbalance vector R. For example, the coil winding pattern may be suchthat the last coils to be wound on the armature tend to be the largestand most massive (e.g., because they at least partly overlie previouslywound coils). These last coils may also tend to have the largest momentarms d in equations (1) and (2). Thus both variable m and variable d maybe greatest for these coils. A given magnitude of vector R may thereforebe offset with a smaller difference in number of wire turns in thesecoils than in any previously wound pair of coils. In addition, becausethese coils are wound last, they can be of different sizes withouthaving any secondary effects on the sizes, shapes, or balance of anyother coils.

If it is desired to base the angular orientation of all coils on thedirection of vector R as described above, assembly 10 can be rotated inapparatus 100 so that marking tool 130 marks the vector R data onlamination stack 14 at the angular location where that data can be readby sensor 220 when apparatus 200 has rotated assembly 10 to the positionat which winding must begin in order for the last-wound coils to be inplanes perpendicular to vector R. Again, this is only one example of howthe data regarding the direction of vector R can be communicated towinding machine controls 202 so that winding machine 200 can rotateassembly to the angular position at which coil winding should start sothat the last coils wound are those that are to be used to produce coilunbalance vector F. It will also be appreciated that in other coilwinding patterns it may be desired that particular coils other than thelast-wound coils are to be used to produce vector F. It will be apparentfrom the foregoing how the systems of this invention can use thedirection of vector R to cause winding machine 200 to rotate assembly 10to a coil winding start position such that, when the desired coils arebeing wound, vector R has a desired orientation relative to those coils(e.g., vector R is substantially perpendicular to the planes in whichthose coils are being wound).

FIG. 10 shows armature 10' after it has been completely wound. It willbe apparent from this FIG. that the armature is statically balancedbecause the magnitudes of vectors R and F are equal and their directionsare opposite to one another. However, as FIG. 10 also shows, vectors Rand F may not be in the same plane perpendicular to axis 22. The axiallocation of vector F tends to be always at the axial center oflamination stack 14 because the coils are axially centered on thatstack. Vector R, however, has components that may not be axiallycentered on lamination stack 14. For example, any contribution to R fromcommutator 20 will axially displace R toward the commutator. Thusalthough wound armature 10' is statically balanced by R and F, it maynot be dynamically balanced. In particular, when armature 10' is rotatedat high speed, as when it is used in a motor, the axial offset between Rand F produces a couple in the plane defined by vectors R and F. Thiscouple can eliminated by a dynamic balancing operation of this inventionas will now be described.

The dynamic unbalance characteristics of wound armature 10' can bedetermined by placing armature 10' in unbalance measuring apparatussimilar to that shown in FIGS. 6 and 7. In this case, however, thedynamic unbalance characteristics are determined from differences in themotions of floating supports 102a and 102b as armature 10' is rotated inthe unbalance measuring apparatus. Once the characteristics of thedynamic unbalance have been determined, the armature is placed inanother machine (indicated by resin dispensers 300a and 300b in FIG.10). (Any of the techniques discussed above for transfer of unbalanceinformation from machine 100 to machine 200 can be used for transferringdynamic unbalance information from the apparatus which measures dynamicunbalance to the apparatus which includes resin dispensers 300.) Each ofdispensers 300 can add mass (e.g., a quantity of a dense resinous gel)to a respective opposite axial end of the coils wound on laminationstack 14. In particular, dispenser 300a adds gel mass 302a to armature10' in plane 40a, while dispenser 300b adds gel mass 302b to armature10' in plane 40b. The apparatus which includes dispensers 300 rotatesarmature 10' about axis 22 so that gel masses 302 are deposited ondiametrically opposite sides of the armature, preferably in the planedefined by vectors R and F. The positions of gel masses 302 relative toaxis 22 are such that when armature 10' is rotated about axis 22, thecouple produced by masses 302 is opposite to the couple associated withvectors R and F. In addition, the magnitudes and radial locations ofmasses 302 are such that the magnitude of the couple produced by masses302 is equal to the magnitude of the couple associated with vectors Rand F. The couple of masses 302 therefore cancels the couple of vectorsR and F, and armature 10' is dynamically balanced.

Masses 302 are preferably equal to one another and equally spaced fromaxis 22 so that they do not disturb the static balance of the armature.If R is the magnitude of vector R (or the magnitude of equal andoppositely directed vector F) and s is the axial spacing between vectorsR and F, then to achieve dynamic balance the mass M of each mass 302should be determined by the equation:

    M=Rs/rS                                                    (3),

where r is the radial distance from axis 22 to either of masses 302, andS is the axial distance between those masses (i.e., between planes 40).Each of dispensers 300 is accordingly controlled to dispense a mass M.

Assuming that masses 302 are a resin material generally like thematerial that is typically used to impregnate at least the axial ends ofthe coils, masses 302 may be applied during the usual resin impregnationoperation or immediately after that operation. Preferably, masses 302are applied prior to the operation by which the impregnation resin iscured. In this way both the impregnation resin and masses 302 are curedtogether in one curing operation which ensures firm anchoring of masses302.

It will be noted from equation (3) that if the apparatus of FIGS. 6 and7 determines the axial location of vector R, everything necessary tocompute M is known even prior to the coil winding operation. This is sobecause the vector F always has the same axial location (i.e., at theaxial center of lamination stack 14). This makes it possible to omitsubjecting the armature to a second unbalance measuring operation afterwinding.

FIG. 11 is a summary of the steps which can be carried out in accordancewith this invention to produce armatures that are both statically anddynamically balanced. In step 400 lamination stack 14 is formed. In step402 shaft 12 is pressed into stack 14 as shown in FIG. 1o In step 404end fibers 16 are placed on the assembly as shown in FIG. 2. In step 406commutator 20 is placed on the assembly as shown in FIG. 3. In step 408assembly 10 is tested as shown, for example, in FIGS. 6 and 7 todetermine the magnitude, direction, and axial location of unbalancevector R. In step 410 assembly 10 is wound with coils as shown, forexample, in FIGS. 8 and 9. This winding operation includes adjusting thenumber of wire turns in certain coils to compensate (in a static balancesense) for vector R. Unbalance information from step 408 is accordinglyemployed in step 410. In step 412 the coils are impregnated with resinin the conventional manner. In step 414 gel masses 302 are added to thewound armature as shown, for example, in FIG. 10 to substantiallyeliminate any dynamic unbalance of the armature. Again, unbalanceinformation from step 408 is used in step 414. (This unbalanceinformation may be transferred from step 408 to step 414 by any of thetechniques discussed above for transfer of such information from step408 to step 410.) Finally, in step 416 the resin material applied insteps 412 and 414 is all subjected to a curing operation.

It will be understood that the foregoing is only illustrative of theprinciples of this invention, and that various modifications can be madeby those skilled in the art without departing from the scope and spiritof the invention. For example, masses 302 can alternatively be appliedin a manufacturing stage other than that described above (e.g., afterthe impregnation resin has been cured). Whenever masses 302 are applied,however, there should thereafter be a curing operation which causesthose masses to harden and permanently adhere to the armature.

The invention claimed is:
 1. A method for producing an armature for adynamo-electric machine, said armature having a central longitudinalshaft, a substantially cylindrical core mounted substantiallyconcentrically on said shaft, and a plurality of coils of wire wound onsaid core, said coils of wire being made up of turns of wire which aresubstantially disposed in planes that are substantially parallel to butradially spaced from the central longitudinal axis of said shaft, saidmethod comprising the steps of:measuring any initial unbalance of saidarmature about said central longitudinal axis prior to winding at leastsome of said coils; and winding at least one of said coils afterperforming said measuring step with a number of turns of wire that isselected to unbalance said coils about said central longitudinal axis sothat the unbalance of said coils at least partly cancels said initialunbalance, wherein said measuring step comprises the steps of: measuringthe magnitude of said initial unbalance; and determining the radialdirection of said initial unbalance, and wherein said winding stepcomprises the step of: winding a first coil which has turns of wiresubstantially disposed in a first plane that is radially spaced fromsaid central longitudinal axis in a direction opposite to said radialdirection with a greater number of turns of wire than a second coilwhich has turns of wire substantially disposed in a second plane that issubstantially parallel to said first plane but radially spaced from saidcentral longitudinal axis in said radial direction.
 2. The methoddefined in claim 1 wherein said first and second planes areapproximately equally radially spaced from said central longitudinalaxis.
 3. The method defined in claim 2 wherein said greater number ofturns times the mass of the wire in each of said turns times the radialdistance between said central longitudinal axis and said first plane isapproximately equal to said magnitude of said initial unbalance.
 4. Amethod for producing an armature for a dynamo-electric machine, saidarmature having a central longitudinal shaft, a substantiallycylindrical core mounted substantially concentrically on said shaft, anda plurality of coils of wire wound on said core, said coils of wirebeing made up of turns of wire which are substantially disposed inplanes that are substantially parallel to but radially spaced from thecentral longitudinal axis of said shaft, said method comprising thesteps of:measuring any initial unbalance of said armature about saidcentral longitudinal axis prior to winding at least some of said coils;winding at least one of said coils after performing said measuring stepwith a number of turns of wire that is selected to unbalance said coilsabout said central longitudinal axis so that the unbalance of said coilsat least partly cancels said initial unbalance; measuring any dynamicunbalance of said armature about said central longitudinal axis; andadding mass to said armature in order to at least partly cancel saiddynamic unbalance.
 5. The method defined in claim 4 wherein said step ofmeasuring dynamic unbalance comprises the steps of:aligning said centrallongitudinal axis with a reference axis; rotating said armature aboutsaid central longitudinal axis; and detecting any tendency of saidcentral longitudinal axis to deviate laterally from alignment with saidreference axis.
 6. The method defined in claim 5 wherein said step ofadding mass comprises the steps of:adding a first mass to said armatureat a first predetermined location which is radially spaced from saidcentral longitudinal axis in a first radial direction; and adding asecond mass to said armature at a second predetermined location which isradially spaced from said central longitudinal axis in a second radialdirection which is opposite to said first radial direction, said firstand second masses being axially spaced from one another parallel to saidcentral longitudinal axis.
 7. The method defined in claim 6 wherein saidfirst and second masses are sized and radially spaced from said centrallongitudinal axis so that they do not affect static balance of saidarmature about said central longitudinal axis.
 8. The method defined inclaim 7 wherein said first and second masses have approximately the samesize and radial spacing from said central longitudinal axis.
 9. Themethod defined in claim 7 wherein said first and second masses are bothdisposed on said armature substantially in a plane defined by saidcentral longitudinal axis and said radial direction of said initialunbalance.
 10. The method defined in claim 7 wherein said step ofmeasuring dynamic balance comprises the step of:determining the axialcenter of said initial unbalance.
 11. The method defined in claim 10wherein said first and second masses are such that the sum of theirmasses times one-half the axial spacing between them is approximatelyequal to said magnitude of said initial unbalance times the distancealong said central longitudinal axis between the axial center of saidcore and said axial center of said initial unbalance.
 12. The methoddefined in claim 6 wherein said step of adding mass comprises the stepof:applying a body of a resinous material to said coils at apredetermined location adjacent an axial end of said core.